Infrastructure change to come in a separate commit.
https://mail-index.netbsd.org/tech-crypto/2017/05/06/msg000719.html
Patch for CVE-2018-049.
Prevent shell execution with r command.
Check bounds before dereferencing in encryption routines.
Document -S to disable ! commands.
Sync with NetBSD-8
closes#268
Change-Id: I1c2849e0097b0cc9f89beef5ee24ccd9d73b4ee2